This invention relates to systems and methods for obtaining spin and mechanical twist data for an optical fiber. Specifically, the systems and methods of the present invention measures the rate of rotation imparted into a molten fiber as well as the rate of mechanical rotation introduced on a cooled fiber as it is drawn from a preform. This application is being filed concurrently with application Ser. No. 10/202,540 entitled Systems And Methods For Forming Ultra-Low PMD Optical Fiber Using Amplitude And Frequency Keyed Fiber Spin Functions, which is incorporated by reference into this application.
It is well known that the so-called xe2x80x98single mode optical fiberxe2x80x99 commonly used in communications systems is not completely symmetric in its cross-sectional geometry. Rather, the cross section of a typical optical fiber is slightly elliptical in shape and can be attributed in part to various factors present during manufacturing. This imperfection contributes to the degradation of the signals carried by optical fibers, particularly at high transfer speeds. One type of degradation is called polarization mode dispersion (PMD). PMD relates to the two orthogonal modes of light rays that propagate through the fiber. Because of the non-circular shape, as well of other imperfections and internal stresses, these two modes may propagate with different propagation constants. The difference between the propagation constants is called birefringence.
Various attempts to reduce PMD have been made and involve imparting a rotation into the fiber during manufacturing. One prior art method of reducing PMD involves spinning the preform during the fiber drawing process. See, for example, Barlow, et al., Applied Optics, 20:2962-2968, 1981; Payne, et al., IEEE Journal of Quantum Electronics, QE-18:477-487, 1982; Rashleigh, xe2x80x9cFabrication of Circularly Birefringent Single Mode Fibers,xe2x80x9d Navy Technical Disclosure Bulletin 5:7-12, 1980; and PCT Patent Publication No. WO 83/00232. The spinning causes the internal geometric and/or stress asymmetries of the fiber to rotate about the fiber""s axis. By performing the spinning during drawing, i.e., when the root of the preform is substantially molten, essentially pure rotation is performed on the fiber asymmetries, as opposed to a combination of rotation of the asymmetries and the introduction of rotational stress as would occur if the fiber were twisted after having been drawn. For a discussion of the use of twist to reduce PMD, see, for example, Schuh et al., Electronics Letters, 31:1172-1173, 1995; and Ulrich, et al., Applied Optics, 18:2241-2251, 1979.
However, spinning the molten preform presents operational difficulties. Given the desired draw rate, the semi-solid/semi-molten preform must be spun at a high rate of rotation (several thousand r.p.m.). Alternatives have been developed which rely on manipulating the fiber as it is drawn, and keeping the preform stationary. A rotation can be imparted into the fiber by altering the angle of take-up rollers, which pull the fiber from the preform. See, for example, U.S. Pat. No. 5,298,047. Regardless of which method is used, a spin is imparted into the optical fiber and the amount of spin imparted impacts the degree to which PMD is reduced. When a spin is xe2x80x9cimpressedxe2x80x9d on the fiber, the fiber material in the hot zone is torsionally deformed and results in the deformation being xe2x80x98frozenxe2x80x99 into the fiber as it cools from its molten state. Once cooled, the fiber exhibits a permanent xe2x80x9cspinxe2x80x9d, i.e., a permanent torsional deformation. The amount of spin that is actually introduced into the molten fiber is not always the same as the amount that is attempted to be introduced. There are various factors affecting the rotational transfer imparted into the fiber. For example, the fiber may xe2x80x98slipxe2x80x99 on the rollers that impart the twist and the length of fiber span between the molten fiber and the spinning apparatus impacts the degree of cooling, and thus the amount of spin actually imparted. Consequently, the spin actually introduced compared to the spin attempted to be introduced is less than 100% and cannot always be predicted at any given moment in time.
Consequently, a mechanism for measuring the actual amount of spin imparted at any moment in time during production is advantageous to verify that the appropriate levels of spin are introduced into the fiber. The existence of the imparted spin can be readily ascertained, e.g., by microscopic examination of bubbles in the fiber to determine rotation of core ovality or eccentricity, or by means of a traveling magneto-optic modulator, as used by M. J. Marrone et al., Optics Letters, Vol. 12(1), p. 60. However, microscopic examination of bubbles in the fiber or similar techniques require destruction of the fiber and are undesirable. Such analysis provides little opportunity to monitor the formation of spin in real time and affect the manufacturing in situ. Further, microscopic examination is both time and labor consuming, in addition to being destructive. Additionally, such examination only allows determination of spin at sampled pointsxe2x80x94determination of spin all along a fiber cable would be impractical using this method and would destroy the optical fiber.
One such mechanism to monitor the spin in real time was proposed by F. Cocchini, A. Mazzotti, A. Ricco, and A. Rossi, On-Line Fiber Spinning Monitoring for Low PMD Optical Fibers, Proceedings of the 49th International Wire and Cable Symposium, Atlantic City, N.J. Nov. 13-16, 2000. This scheme uses real time Fourier analysis of the fiber diameter data as obtained by transverse on-line measurements of the optical fiber shortly after is it drawn from the preform. The system used power spectrum analysis to correlate the signals with the spin rate. The presence of high output frequencies correlates to high spin rates.
However, the use of Fourier transform analysis is complicated and requires significant computer processing, and requires further analysis to correlate the results with the spin rate. Because of the vast amounts of data produced in real time, the processing power required to analyze such results in real time is significant. Thus, while the prior art can measure the spin rate, it may not be practical to perform real-time measurements so as to provide feedback to control systems that affect the spinning apparatus during the manufacturing process. Consequently, a simpler mechanism for determining real time spin rate is needed that requires less processing power and that can determine spin in real time.
In addition to xe2x80x98spinxe2x80x99, another metric regarding optical fibers that is measured during the manufacture is xe2x80x98twistxe2x80x99. The spin and twist metrics are related, and some may use such terms interchangeably. However, as used herein, spin refers to the rotation introduced into the molten optical fiber, whereas as xe2x80x98twistxe2x80x99 refers to the rotation introduced onto the cooled optical fiber. Spin is imparted into the molten fiber, and is permanently fixed when the fiber has cooled. Twist refers to the mechanical rotational force imposed on the optical fiber after it has cooled. Twist introduces torsional stresses on the fiber, and extreme amounts can cause microscopic cracks, and ultimately physical destruction of the fiber. It is desirable to reduce or eliminate twist introduced on the fiber. One method of reducing twist on optical fiber is to xe2x80x98respoolxe2x80x99 the fiber by unwinding it and rewinding the fiber, which is also time consuming and labor intensive. It would be preferable to avoid or minimize the introduction of twist to acceptable levels during manufacturing. In either case, a method of measuring the levels of twist in the optical fiber during manufacturing is required. Therefore, there is a need for systems and methods for measuring twist introduced on optical fibers.
In one embodiment of the invention, a method is disclosed for measuring spin imparted in optical fibers by measuring the diameter of an optical fiber as it is drawn from a molten preform, detecting the periodic minimum and maximum diameters, processing the diameter measurements and the draw rate to determine the spin imparted into the optical fiber.
In another embodiment of the invention, a system is disclosed that measures the spin imparted into an optical fiber comprising an apparatus for drawing fiber from a molten preform, a measurement device measuring the diameter of the fiber as it is drawn, a processor receiving inputs regarding the fiber draw rate and diameter measurements and determining the spin rate in the optical fiber.
In another embodiment of the invention, a method of determining twist on an optical fiber is disclosed by drawing an optical fiber, monitoring the rate of draw of said optical fiber, measuring a first diameter of the optical fiber at a first point as it is being drawn and detecting periodic diameter measurements, and processing said periodic diameter measurements and determining the twist on the optical fiber.
In another embodiment of the invention, a method for measuring rotations along the principle axis in an optical fiber is disclosed where the optical fiber is drawn past a fiber diameter measuring device at a known draw rate, measurements of the diameter of the optical fiber is obtained by the fiber diameter measuring device, periodic changes in the diameter of the optical fiber is determined, and the periodic changes in the measurements is processed in conjunction with said known draw rate to measure the rotations along the principle axis of the optical fiber.